1. Field of the Invention
The present invention relates to a liquid crystal display device, and particularly relates to a novel liquid crystal display device that uses liquid crystal material having an antiferroelectric phase.
2. Description of the Related Art
Liquid crystal displays currently being manufactured principally include multiplex-driven STN (super twisted nematic) liquid crystal displays and TN (twisted nematic) liquid crystal displays that are active-matrix driven by TFT (thin-film transistors). These liquid crystal displays, however, lack sufficient response speed during half-tone display, and this has brought attention to the need for faster devices.
Given this situation, ferroelectric liquid crystal having a chiral smectic C-phase is receiving attention, and surface-stabilized ferroelectric liquid crystal (SSFLC) devices which use ferroelectric liquid crystal are also coming into practical use. Display devices that use this SSFLC are principally limited to switching between the two states of bright and dark, and while they have a memory property, they are not capable of gray-scale display. Methods have been proposed to produce gray-scale display with a SSFLC by controlling multidomains having differing threshold values, but such methods cannot provide continuous gray-scales. Moreover, ferroelectric liquid crystal is not shock-resistant, and even the slightest shock causes variation in the liquid crystal orientation. Some currently commercialized SSFLC displays actually incorporate heaters for initializing a disordered orientation state.
One known means of solving this problem is a liquid crystal display that employs deformed helix ferroelectrics (DHF) such as the devices described in `Advances in Liquid Crystal Research and Applications` by Ostovski et al. (Oxford/Budapest, 1980, p. 469) and in Japanese Patent Laid-open No. 152430/89 (JP, A, 1-152430). A liquid crystal display that employs this DHF mode uses ferroelectric liquid crystal in which the natural helical pitch in a ferroelectric phase (SmC*-phase), which is formed with smectic A-phase molecules at a tilt, is sufficiently short, i.e., shorter than the cell gap d. Due to the sufficiently short helical pitch, the helix is not constrained by surface stabilization in a DHF-mode liquid crystal display. FIG. 1 is a perspective that presents a schematic view of the structure of a conventional ferroelectric liquid crystal display device using DHF mode.
In the liquid crystal display device shown in FIG. 1, liquid crystal layers are sandwiched between a pair of transparent substrates 81 and 82, and transparent electrodes 83 are formed on each of the opposing surfaces of substrates 81 and 82. Liquid crystal molecules 86 within the liquid crystal layers describe cone-shaped loci, and these loci are therefore drawn as cones 84 in the figure. In addition, the direction of spontaneous polarization of liquid crystal molecules 86 is shown in the figure by arrows 85.
As shown in FIG. 1, the orientation of the liquid crystal in DHF-mode liquid crystal display devices is most typically in a `bookshelf` arrangement (layer structure 88), and in addition, is aligned so as to describe helices in a direction parallel to the substrate surface. In such a liquid crystal display device, however, when the helical pitch of the liquid crystal orientation corresponds to the wavelength range of visible light, a stripe pattern appears and a diffraction grating is formed. When the helical pitch is made shorter than the wavelength range of visible light (preferably, shorter than a half-wavelength .lambda./2) this diffraction is minimized, and the apparent refractive index is equalized. In other words, the liquid crystal can be treated in the same way as a medium having single-axis birefringence.
FIGS. 2A to 2E are views illustrating the operation of a DHF-mode liquid crystal display device when treated as a single-axis birefringence medium. In FIGS. 2A and 2B, the axis of transmission of the polarizing plates joined to a pair of transparent substrates is shown as a heavy line for the upper polarizing plate and as a thin line for the lower polarizing plate. FIGS. 2C, 2D, and 2E each show sections of index ellipsoids corresponding to the polarity of the applied voltage. As shown in FIGS. 2A to 2E, a DHF-mode liquid crystal display device treated as a single-axis birefringence medium has single-axis birefringence with the axis in the direction of the helical axis when voltage is not applied. When voltage is applied, however, the liquid crystal display device gradually shifts from the helix arrangement of the liquid crystal orientation to a distorted helix structure, whereby a continuous gray-scale display is produced due to the altered birefringence and changed transmittance. This type of drive method is described in, for example, Japanese Patent Laid-open No. 194625/94 (JP, A, 6-194625).
As described in Y. Suzuki, et al., Electronics (Tokyo) Journal pp. 45-48, March 1994, a liquid crystal display device has also been proposed that switches between three stable states using an antiferroelectric liquid crystal having a chiral smectic Ca phase. FIGS. 3A to 3E show a liquid crystal display device of a prior art that performs switching between three stable states using this antiferroelectric liquid crystal. FIG. 3A is a schematic view, FIG. 3B is a schematic view of the operation as seen from the x-axis direction of FIG. 3A, FIG. 3C is a schematic view of the operation as seen from the positive y-axis direction of FIG. 3A, FIG. 3D is a schematic view of the operation as seen from the positive z-axis direction of FIG. 3A, and FIG. 3E shows the arrangement of the transmission axis of the polarizing plates as seen from the positive z-axis direction of FIG. 3A.
As shown in FIG. 3A, this display device has a construction in which an antiferroelectric liquid crystal is sealed between a pair of substrates 91 and 92, wherein liquid crystal molecules 96 and 97 each describe cones 94 as shown in the figure. When voltage is not applied, the liquid crystal orientation is stabilized by the antiferroelectric states in which adjacent layers are aligned such that spontaneous polarization is mutually canceled out. The arrangement of polarizing plates as shown in FIG. 3E in this state enables the display of black. As an electric field is applied, orientation changes according to whether the electric field is positive or negative as shown schematically in each of FIGS. 3B, 3C, and 3D, whereby transmittance increases. This display device basically involves switching between three stable states, but design of the applied pulses can produce a simulated gray-scale display, although not of absolutely continuous tones. In addition, this device is more shock resistant than devices produced from ferroelectric liquid crystal, and has the characteristic of restoring liquid crystal orientation by itself by means of an applied drive field. FIG. 4 shows the voltage-transmittance characteristic of a liquid crystal display device that performs switching between three stable states using this antiferroelectric liquid crystal when a triangular wave of a frequency of 0.01 Hz is applied. As shown in FIG. 4, the voltage-transmittance characteristic of this liquid crystal display device exhibits a hysteresis characteristic.
As described hereinabove, when a conventional liquid crystal display device that employs the DHF mode is put into actual use, the helical pitch employed must be short enough that the helix is not optically distinguishable, i.e., the helical pitch must be shorter than the wavelength of visible light that is within the range of use. Such an extremely short helical pitch, however, results in an increase in the voltage required for altering the helix structure, i.e., the threshold voltage. The drive voltage must therefore be increased when using the conventional liquid crystal display device that employs the DHF mode. In addition, because ferroelectric liquid crystal is used in cases in which the DHF effect is employed, after-image effects such as `sticking` which are caused by spontaneous polarization will occur depending on the direction of spontaneous polarization in the phase boundary of the aligning film, just as in the case of a liquid crystal display device that uses SSFLC.
On the other hand, antiferroelectric liquid crystal display devices that employ three stable states are not capable of realizing absolutely continuous gray-scales, and in addition, cannot be used in combination with active matrix elements because their applied voltage is high, and moreover, because they have a hysteresis characteristic as shown in FIG. 4.